Don’t know where this speed “in the 50s” comes from. I grew up in rural Michigan in the fifties and got my license in 67. Speed limits in the mid to late fifties on the two lane highways was always 65 daytime, and by the late fifties, the interstates (or their predecessors) were 70 in Michigan and 75 in some other states. My dad was a cautious driver and always held it to two or three mph over the limit, ie 67-68 mph back in the 1950s. Some Western states had no absolute speed limits, and admittedly, some states had terrible roads until the Federal Government kicked in money for road construction. Trucks were relatively low powered compared to today, and there weren’t as many passing zones, so that did, in many cases increase trip times, but it wasn’t because the sped limits were significantly lower.